It just creates a best fit cube around the tiles you've placed and then looks at the floor. So it usually works ok, but you can fool it with tiles sticking out behind walls or odd shaped rooms. Best to just mock things up quickly and check it looks ok.

At the moment seems to be all, time to go to the ghost hack thread and post some more issues XDD thanks for the help all the time, btw ^^ you guys are pretty much the best customer support I've ever had (besides being the game devs XD).